The growing popularity of wireless communication for voice, data, and video applications has created a need for larger wireless capacity.
Current wireless communication systems are based on the Single-Input, Single-Output (SISO) channel where a single transmitter communicates with a single receiver over the channel. In one common arrangement, many mobile devices, such as cell phones, may be in communication with many base stations that are connected to a wired communication system. Either the mobile device or the base station selects a channel over which the mobile device and base station communicates. The selected channel is used exclusively by the specific mobile device and base station until the communication link is terminated by either device. No other device can use the channel while the communication link is established.
In some SISO systems, the selection of the channel is based on the strength of the signal received by the base station. By selecting the channel with the strongest signal, the bit rate of the communication is maximized. In one scenario, the several base stations may receive a signal from a particular mobile device and the base station with the strongest signal will establish a channel with the mobile device.
Multiple-Input, Multiple-Output (MIMO) systems, in contrast, are characterized by base stations and mobile devices having more than one antenna. In a typical situation, the transmitted data stream is demultiplexed into nT separate sub-streams. Each sub-stream is encoded into channel symbols and transmitted over one of the nT transmission antennas. The advantage of the MIMO system over the SISO system is the linear increase in spectral efficiency for MIMO systems compared to the logarithmic increase in spectral efficiency for SISO systems as nT increases.
The single-user MIMO link may be described by the equation:y=Hx+n  (1)where y is a nR×1 receive vector, x is a nT×1 transmit vector, n is a nR×1 additive white Gaussian noise (AWGN) vector, and H is a nR×nT channel matrix. An element in the channel matrix is denoted by hij and represents the complex gain of the channel between the j-th transmitter and the l-th receiver. In most situations, H may be assumed to be random and memoryless. It is also assumed that the receiver has perfect channel knowledge. The maximum channel capacity is attained when the transmitter also has perfect channel knowledge and transmits over k antennas, where k is the rank of the channel matrix.
The requirement of perfect channel knowledge cannot be met with current deployable systems. Therefore, there is a need for MIMO systems that can operate efficiently under a feedback capacity constraint.